A bus, also known as an electrical or communications bus, is a physical electrical interface where more than one device shares the same electric connection. Buses are common in information technology systems, telecommunication systems, commercial electronics, and vehicular systems. Twisted pair cables are two-wire electrical buses and are particularly suitable for use in vehicle systems as they are more robust and simple to install and configure. Various communication protocols may be used on such electrical buses and used to provide communications between electronic components located throughout a vehicle. In particular, airbag systems have many types of components that may be connected to a network. Airbag systems require robust and failsafe communications between the various components to avoid critical system failures or incorrect airbag deployment.
Distributed System Interface (DSI) message protocol is a single master device multiple slave device data communications (electrical) bus implemented on two wires. The bus utilizes voltage modulation (pulse-width modulation) signaling for digital messages sent from the single master device (master) to the slave devices (slaves) and current mode signaling from the slave devices to the master device. Slave devices only transmit in response to messages sent from the master device and are synchronized to the master signals. The DSI message protocol has many versions, including DSI2 and DSI3. Further, buses operating under other protocols, such as the Peripheral Sensor Interface 5 (PSI5) and Local Interconnect Network (“LIN”) are common buses providing communication between electrical components of vehicles. LIN is a serial network protocol used for communication between components in vehicles.
According to the DSI message protocol, slave responses to commands are sent using a modulated current signal, which is self-synchronized to a falling edge voltage from the master voltage signal. Master and slave devices transmit simultaneously, i.e. the DSI is a duplex system of communication. During the response time the master device sends a pulse train of any combination of ones or zeros. The current mode bits are sent during the bit time and sampled by the master device at the falling edge of the voltage pulse generated by the master device. When the slave device produces a logical one, the slave device draws additional current above a particular threshold from the electrical bus during the bit time. If a logical zero is transmitted the slave device does not draw additional current during the bit time. The master device detects the current drawn from the electrical bus at a predetermined point in the bit time, i.e. the sampling point, to determine the bit type transmitted by the slave device.
In the prior art, when the master voltage pulse is generated, some current flows from the master device to ground through a set of distributed or parasitic capacitors. The capacitors are stray parasitic capacitors coupled between the pair of wires and the chassis. This current loop acts as an antenna whose radiative emissions exceed the protocol limits. This is particularly critical for single-ended buses such as DSI3, PSI5 and LIN where the common mode is proportional to the signal. Current PSI5 transceivers exceed the emission limits because of prior art master to slave communications. The master voltage pulse typically takes the form of a square wave with constant rising and falling edges when transmitted over the bus. The master voltage pulse may also take the form of a trapezoidal wave with constant edge slopes.
Methods to reduce the radiation emissions of the prior art typically involve the use of complex look-up tables or require signal filtering. Signal filtering to reduce the radiation emissions requires adjustments to the filtering based upon varying frequency while the use of look-up tables is limited to digital signal generation. Therefore, an improved method and system for the reduction of radiative emissions during digital and analog bus signal transmissions is desired.
The figures are not intended to be exhaustive or to limit the invention to the precise form disclosed. It should be understood that the invention can be practiced with modification and alteration, and that the invention be limited only by the claims and the equivalents thereof.